The ultimate goal of this proposal is to fully characterize dentin and modify it to enhance adhesion to it. The emphasis in the next five years will be on expanding our efforts beyond normal circumpulpal dentin to include other types of dentin. Normal dentin structure exhibits variation in structure and properties with location, but there are also many dentins, including primary, secondary, tertiary, coronal, root, sclerotic, transparent, carious, demineralized, remineralized, and hypermineralized. They reflect alterations in the fundamental components of the structure as defined by changes in their arrangement, interrelationships or composition. Several have important implications for our ability to develop long lasting adhesion or bonds to this structure. Progress in diverse areas including adhesion to dentin and dentin physiology require a thorough knowledge of the basic microstructure, composition and components of dentin. In the prior period, we studied normal dentin as a complex composite material that needed ot be understood from the materials science perspective. We developed some unique methods for its study, including x-ray tomographic microscopy, atomic force microscopy, and atomic force microscopy nanohardness. In addition, we began the process of defining some of its important compositional, mechanical and physical properties as related to microstructure. Although significant progress has been made, we have evaluated only one dentin type, and the knowledge gained will form a critical reference base for understanding other types. Thus in the next period we will focus on important variations of dentin which are encountered in many clinical situations. The program project will encompass four projects. The first, Characterization of Normal and Altered Forms of Dentin, is a comprehensive investigation of different types of dentin to understand their microstructures and composition on a microscopic level. We will use many of the same analytical methods as in the last project period, such as scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDS). Fourier transform infrared spectroscopy (FTIR), x-ray tomographic microscopy (XTM), atomic force microscopy (AFM), and x-ray diffraction (XRD). The second project, Mechanical Properties and the Structure of Dentin, will define the mechanical properties of the constituent parts of dentin (i.e. peritubular, intertubular) and how they vary with intratooth position, age, or gender. Nanohardness and modulus will be measured using the atomic force microscopy. The third project, The Resin-Dentin Interdiffusion Zone, will test the hypotheses that the resin-dentin interdiffusion zones formed by two types of adhesives have the same substructure and that the substructure does not vary with dentin depth or altered form of dentin.